This invention relates to driveshafts for providing a drivable rotary connection between two components and, in particular, to a driveshaft assembly for use as a motor vehicle half-shaft drivably connecting a driven road wheel of a motor road vehicle to a source of motive power.
The increased use of engines having a lower number of cylinders such as, for example two and three cylinder engines has increased the significance of firing frequency torsional vibrations to passenger comfort. This is especially the case when damping technologies such as dual mass flywheels and/or balancing shafts are excluded for cost reasons.
Low stiffness for the driveshafts is required to isolate vehicle occupants from the torsional engine vibrations under low RPM, high torque driving referred to as ‘lugging’. During low RPM, high load driving, engine main order firing frequency vibrations at 40-50 Hz are transmitted through transmission mounts to vehicle occupants via the excitation of cavity modes in large panels and as vibration though driver controls and seats.
Reducing driveshaft stiffness reduces the motion of lugging vibration at the compliances, primarily engine and chassis mounts thereby reducing the amplitude of vibration transmitted to the cabin, improving passenger comfort.
Driveshafts with high stiffness are beneficial for reducing low frequency (2 to 8 Hz) longitudinal vehicle acceleration noise or ‘shuffle’. This fore-to-aft oscillatory motion can be uncomfortable and cause motion sickness for occupants. Shuffle is created by torsional winding and unwinding of the less stiff driveline components under the forces exerted on them by the vehicle mass and torque source during transient maneuvers.
Increased driveshaft stiffness both improves the damping ratio by increasing the motion at compliances, primarily tires, and increases the vehicle natural shuffle frequency. This reduces both time for the amplitude of oscillation to drop below a perceivable threshold and passenger exposure to less comfortable low frequencies. Increased shuffle frequency and system damping also enables calibratable drivability methods to be employed less aggressively, improving the connectivity and immediacy of the vehicle response to the driver's acceleration request.
Therefore conflicting requirements exist for a driveshaft in that it is not possible to optimally absorb high frequency (40-50 Hz) and transmit low frequency (2-8 Hz) motion with a driveshaft having a single torsional stiffness.